The present invention relates to ice making machines and particularly to control methods for automatic ice making machines.
Numerous automatic ice making machines have been developed over the years. Most of these machines have been free-standing units that are connected to electrical and water supplies and make ice using a standard refrigeration system. The ice machines often have a control system which automatically operates the machine through freeze and harvest cycles, and which turns the machine off when sufficient supplies of ice have been made.
Such ice machines come in all sizes, from large machines that make hundred of pounds of ice in an hour, to smaller machines which make a few pounds of ice an hour, the control systems for such machines vary from sophisticated to simple.
Many cube ice making machines use a hot gas bypass valve to harvest the cube ice by sending hot refrigerant from a compressor directly to an evaporator mounted on the back of a cube forming evaporator plate. Instead of freezing water into ice, the evaporator then melts the ice. Knowing when to start and end the harvest cycle is important. The maximum efficiency of the machine requires that the harvest cycle be started when ice has formed sufficiently, and stopping the harvest cycle as soon as the ice is released from the ice forming evaporator plate. Prior art patents disclose the use of ice thickness sensors to initiate a harvest cycle, and an electro-mechanical sensor, such as a water curtain switch, to detect when the ice cubes fall off of the ice-forming evaporator plate. There are numerous other control sensors and mechanisms to start and stop the harvest cycle.
One problem with many of the sophisticated control systems is that they require components that add significant cost to the ice making machine. On relatively small ice machines, where the manufacturing cost is minimized, a trade off is made in that the control system does not operate the machine in the most efficient manner. For example, in some ice machines, the durations of the freeze and harvest cycles are based on a sensor which measures the temperature or pressure of the refrigerant on the suction side of the compressor. Other systems use a thermostat on the evaporator or outlet of the evaporator. In these systems, when a predetermined temperature is reached, the machine changes to a harvest cycle, and when another temperature is reached, they change back to a freeze cycle. When the ambient air is warmer, the freeze cycle duration is longer. Some such systems include an adjustment knob so that the cycle time can be increased or decreased as desired if ice cube thickness is too great or too small.
One problem with such a simple control system is that it does not automatically take into account several variables. For example, the optimum freeze and harvest cycle durations will depend not only on ambient air temperatures, but on such factors as how clean the condenser is, and whether any foreign objects are blocking the flow of air past the condenser. The adjustment knob can be used to adjust the cycle times as these factors change, but this often requires a service technician, or is not done properly. As a result, the machines may not produce sufficient ice, and they have higher operating costs than necessary.
U.S. Pat. Nos. 5,182,925 and 5,291,752 to Alverez et al. disclose an ice machine that starts the harvest cycle when enough of a batch of water initially charged to a reservoir has frozen into ice to trip a low water sensor. A thermistor located at the outlet of the condenser is used to end the harvest cycle. The temperature of refrigerant is measured by the thermistor at the beginning of the harvest cycle to get an idea of how hot the refrigerant is that is passing through the hot gas defrost valve. A microcontroller then determines what the temperature of the refrigerant out of the evaporator should be when the harvest cycle is complete. A second thermistor on the outlet side of the evaporator is monitored and when this temperature is reached, the system ends the harvest cycle and returns to the freeze cycle. Alternatively, the microcontroller sets a time for the harvest to last. In yet another alternative, the microcontroller looks at the rate at which the refrigerant exiting the evaporator rises, and when a substantial rise is detected, terminates the harvest cycle.
This control mechanism has several drawbacks. First, it requires a variety of sensors, including a low water level sensor and two thermistors. Second, the thermistor located on the exit side of the evaporator is located where it has to be protected from water condensation on the cold refrigerant return line and is subject to vibrations from the compressor, which is also connected to this line. Third, the time period at which the thermistor senses the temperature of the refrigerant leaving the condenser is right after the harvest cycle commences, which is a relatively unstable time period during the refrigeration cycle which makes consistency of operation more difficult.
It would be of great benefit if a simple control mechanism could be developed which could initiate a harvest cycle without the use of a water level sensor or ice thickness sensor, both of which are subject to failure after repeated use in conditions to which they are typically exposed. Also, it would be beneficial if an inexpensive control system could be developed that could be used on small ice machines that would not add much to their manufacturing cost but which could greatly improve the efficiency of the machine compared to simple control systems known heretofore. Preferably such an improved control system would start and stop the harvest cycle dependent on varying conditions, including not only ambient temperature, but increasing amounts of dirt on condenser coils and partial blockage of air flow past the condenser coil.